The present invention relates to a vertical distillation column for distilling or fractionating a feed under vacuum having a feed inlet arranged in the lower part of the column, and a de-entrainment device arranged in the column between the feed inlet and the gas outlet.
During normal operation the gas outlet is connected with a vacuum device, for example a stream ejector, to maintain the pressure in the top of the vertical distillation column between 100 Pa (absolute) and 10,000 Pa (absolute), and feed comprising gas and liquid is introduced into the column via the feed inlet. Since the gas will not totally entrain the liquid, a part of the liquid drops into the lower part of the column and is discharged therefrom through a liquid outlet arranged in the lower part of the column. A mixture of gas and remaining liquid is passed upwards. To remove part of the liquid from the mixture, the known vertical column is provided with a de-entrainment device in the form of a horizontal layer of structured or random packing.
Liquid collected on the packing pertaining to the de-entrainment is collected, which liquid flows downwards and drops from the de-entrainment device into the lower part of the column from where it is discharged through the liquid outlet. Gas having a reduced content of liquid is passed from the de-entrainment device to the upper part of the column where it can be treated, for example a condensable component can be separated from the gas in a condensing section.
In the specification, the term gas also refers to vapours or to mixtures of gases and vapours.
As the amount of feed introduced into the vertical distillation column per unit of time is increased, the velocity of the gas flowing through the de-entrainment device is increased as well. A further increase in the amount of feed introduced per unit of time may result in a gas velocity which is so high that liquid collected on the de-entrainment device will be re-entrained, which deteriorates the performance of the de-entrainment device.
The area of the de-entrainment device perpendicular to the direction of the gas flow could be increased to reduce the velocity of the gas flowing through the de-entrainment device, but this area is limited by the diameter of the vertical distillation column.
An important parameter for the performance of the vacuum distillation column is the resistance against flow of gas through the de-entrainment device. This resistance is usually referred to as pressure drop across the de-entrainment device.